1. Field of the Invention
This invention generally relates to polymers having fluorescent colorants. More particularly, the invention relates to articles having fluorescent properties and being composed of multiple layers, which together provide important properties. Such properties provide desired brightness and chromaticity, which shows excellent resistance to weathering and/or overall color durability.
2. Description of Related Art
Articles incorporating fluorescent dyes into polymeric matrices are extensively known in the art for various applications including signage, vehicle markings, roadway markings, and other applications where high visibility is desired and beneficial for any number of reasons, including safety, information dissemination, visibility, visual signaling, and quick detection. The extraordinarily bright appearance of fluorescent materials is what provides this enhanced visibility, which is especially pronounced at dawn and dusk. In some applications, it is important to meet and maintain certain color standards and/or certain durability standards.
Often these polymer systems containing fluorescent colorants are structured in the form of a sheeting, which exhibits fluorescing properties. Particularly suitable applications for these types of films loaded with fluorescent colorants are in connection with uses where signaling is a primary function of the article. Typically, these take the form of signage, which can benefit by exhibiting fluorescing action. Traffic safety and informational signs have been known to incorporate films having fluorescent colorants, which enhance visibility of the signs. Certain types of signage need to have long-term outdoor durability, which is a big hurdle because most fluorescent colorants have poor ultraviolet light stability. Some of these articles incorporate retroreflective features.
Over the years, the art has developed within the field of retroreflective articles. Generally speaking, there are three main types of retrorefelctive sheetings in the traffic industry, i.e. enclosed lens sheeting, encapsulated lens sheeting, and prismatic sheeting. Palmquist U.S. Pat. No. 2,407,680 illustrates so-called enclosed lens retroreflective sheeting articles. Assemblies of this type are also known as engineering grade, utility grade or super engineering grade products, and they have a typical coefficient of retroreflection at a −4° entrance angle and at a 0.2° observation angle between 50 to 160 cd/lx/m2 for white sheeting, depending upon the specific product.
McKenzie U.S. Pat. No. 3,190,178 generally illustrates so-called encapsulated lens retroreflective articles. This includes sheeting of beads encapsulated into polymer, at times referred to as high intensity products. For white sheeting, these have a typical coefficient of retroreflection of about 300 cd/lx/m2. 
A third general category of retroreflective sheeting incorporates microprismatic optical elements which provide exceptional reflectivity, typically between about 400 and about 1600 cd/lx/m2 depending upon the specific product construction and geometry of the cube corner elements. Cube corner retroreflective sheetings are described in Rowland U.S. Pat. No. 3,684,348, Hoopman U.S. Pat. No. 4,588,258, Burns U.S. Pat. No. 5,605,761, and White U.S. Pat. No. 6,110,566. Publications such as Rowland U.S. Pat. No. 3,810,804, and Pricone U.S. Pat. No. 4,601,861 and U.S. Pat. No. 4,486,363 illustrate the manufacture of articles of this type. It will be noted that the art includes retroreflective sheeting by which thermoplastics are embossed into prismatic sheeting. The present invention finds application in products having these principal types of retroreflective construction.
There is also art that teaches how to enhance the UV light durability of retroreflective sheeting which incorporates fluorescent colorants. Some of this art teaches the use of an ultraviolet (UV) light screening layer over or in front of a fluorescent layer. This art includes Japanese Patent Publication No. 2-16042 (Application No. 63-165914) of Koshiji, Phillips PCT Publication No. WO99/48961 and No. WO00/47407, and Pavelka U.S. Pat. No. 5,387,458. The Japanese Publication indicates that UV additives are useful to protect fluorescent sheeting. The PCT publications relate to fluorescent polyvinyl chloride (PVC) film with a UV light screening layer having UV additives, which screen 425 nanometers (nm) and lower. This U.S. Pat. No. 5,387,458 incorporates a UV screening layer for a film of selected polymers containing selected fluorescent dyes.
The art also recognizes other methods of enhancing the durability of fluorescent colors by using stabilizers of the hindered amine light stabilizer type (HALS type). Art in this area includes Burns U.S. Pat. No. 5,605,761 and White U.S. Pat. No. 6,110,566. The former proposes the combination of particular fluorescent dyes and HALS in a polycarbonate matrix. The latter proposes low molecular weight HALS and a thioxanthene dye within a solventless PVC resin.
All of these patents, other art and patent publications, and any others identified herein, are incorporated by reference hereinto.
To a certain extent, art of this type recognizes that making retroreflective signs fluorescent provides enhanced visibility under most lighting conditions. The characteristic bright color and/or the fluorescing characteristics of fluorescent materials attract ones eye to the fluorescent signage or other article. For example, outdoor signage articles, which are colored with fluorescent colorants, enhance visual contrast, making the materials more conspicuous than non-fluorescent colors. When such signage is intended for outdoor uses, two major hurdles are encountered. One is durability under outdoor conditions, and the other is the availability of specific colors.
Unfortunately, most fluorescent colorants have poor UV light stability. When exposed to sunlight or other sources of UV light, fluorescent colorants can fade very quickly. This especially creates problems for traffic and roadway signing applications because the rapid fading of the fluorescent color can dramatically shorten the life of the sign. Although some fluorescent colorants have better UV light stability than others, even the best fluorescent colorants available on the market are not suitable for the extended outdoor durability requirements of a traffic signing application when used alone in a polymeric matrix layer to create a fluorescent retroreflective film. To extend the durability of such films, additional steps must be taken to protect the fluorescent colorants.
A common practice directed toward enhancing outdoor durability is using a UV screening layer such as that taught by the art noted above in an attempt to protect the base fluorescent polymeric matrix layer. Traditionally, such a UV light screening layer is made by dissolving UV light absorbing compounds into a transparent polymeric matrix. The art discloses fluorescent articles consisting of a UV light screening layer deposited in front of a fluorescent color layer. The UV screening layer is intended to absorb a defined range of UV light. UV light has a wavelength range of from 290 nm to 380 nm. Certain art also suggests screening some portion of light in the visible range, such as up to about 400 nm or 410 nm. Often, approaches such as these fail to consider and/or address potential interaction between the UV absorber in the screening layer and the fluorescent dye within the underlying colored layer.
While UV screening is intended to address the outdoor durability problem, several difficulties can arise. One concern is that the UV light absorbing compounds of these screening layers can leach out with time or can diffuse or migrate into the underlying fluorescent layer. This diffusion can actually accelerate fading of the fluorescent colorant in certain instances.
Art such as Burns U.S. Pat. No. 5,605,761 and White U.S. Pat. No. 6,110,566 propose fluorescent sheeting articles of these patents which do not necessarily incorporate a separate UV screening layer. Typically, these teach particular combinations of polymers and fluorescent dyes, often together with HALS materials, in the same film. In particular, the former patent discloses fluorescent articles comprising fluorescent dye and HALS within a polycarbonate matrix. The latter patent purports to teach that the combination of a fluorescent thioxanthene dye and a HALS material in a solventless PVC matrix enhances light stability of the fluorescent colors in the PVC system.
It is also known in the art that certain polymeric matrixes are more suitable as a host for fluorescent dyes with respect to UV light durability of the resulting article. However, acrylic polymers, such as polymethylmethacrylate (PMMA), are generally not known in the art to be a suitable polymeric matrix for fluorescent colors where outdoor light durability is required. For example, Pavelka U.S. Pat. No. 5,387,458 discloses fluorescent articles comprising fluorescent dyes dispersed in various polymeric matrices. This teaches that fluorescent durability of fluorescent dyes in PMMA is poor even with a UV screening overlayer. Burns U.S. Pat. No. 5,605,761 discloses fluorescent articles comprising specific fluorescent dyes and a HALS compound in both polycarbonate and PMMA. This patent teaches incorporation of the HALS compound into the polycarbonate matrix significantly increases the fluorescent durability of the resulting articles, but does not have the same effect with PMMA. Art references such as these conclude that PMMA is not a suitable polymer matrix for fluorescent dyes because such acrylic based articles do not exhibit good fluorescence durability when exposed to extended outdoor weathering.
The conclusion that acrylic is not a suitable host for fluorescent colors is unfortunate because acrylic polymers have advantages over polymers such as polycarbonate. Compared to other polymers such as polycarbonate, such acrylics are inexpensive, easier to process due to a relatively low glass-transition temperature, and typically exhibit better UV light stability. For example, after a few years of outdoor exposure, polycarbonate can exhibit chalking and cracking and can develop a hazy and/or yellow appearance. Acrylics, however, can withstand such outdoor weathering for a significantly longer time before the development of such defects. The primary downside to utilizing acrylic polymers, however, is that acrylics tend to be more brittle than other polymers, such as polycarbonate.
At the present state of the art, although fluorescent acrylic articles appear to hold some promise, issues concerning color stabilization and/or fluorescent stabilization against ultraviolet and visible light radiation present a problem of substantial proportions. Ideally, if a solution could be found, the processing and cost saving benefits of utilizing an acrylic polymer can be realized. Additionally, since acrylic materials will naturally weather better than other polymers, such a solution is potentially all the more important and valuable because an additional UV-light protective cap would not be necessary.
Turning now to the problem of providing articles, which comply with coloration standards, requirements, or needs, coloration considerations present a formidable challenge to suppliers of fluorescent articles, especially those articles that also must be very durable. This is the case whether addressing governmental coloration regulations or other industry standards.
In this regard, it is suggested here that there are three basic approaches for obtaining a desired fluorescent color in the typical instance when a given loading of available fluorescent dyes does not achieve the target fluorescent coloration. One approach is to adjust the loading quantity of the colorant. Often this solution is simply not adequate because the hue of the resulting article will not substantially change.
A second approach is to blend multiple fluorescent dyes together. Such an approach can raise serious compatibility issues, both between the dyes themselves and between one or both of the dyes and the polymer matrix within which they would be loaded. Different dyes have different compatibility with different polymers due to differences between or among their chemical structures. Thus, the UV light durability of a given fluorescent colorant will be different in different polymer matrices. Even if the desired fluorescent color is obtained by blending multiple fluorescent dyes together into a single polymeric matrix, the desired light durability may not be achieved if one of the fluorescent dyes fades more quickly than the other fluorescent dyes in the polymeric matrix. Similarly, one fluroescent dye may have unfavorable interactions with another dye within a polymer matrix. Even if UV light stability can be achieved in a given polymeric matrix when the fluorescent dyes are used alone, the compatibility issues between the dyes can cause the resulting article to have poor UV light stability when these same dyes are blended together into the same polymeric matrix.
It should be noted that art such as Burns U.S. Pat. No. 5,672,643, U.S. Pat. No. 5,674,622, U.S. Pat. No. 5,754,337 and U.S. Pat. No. 5,920,429 suggest making fluorescent yellow articles by blending orange-shade or red-shade perylene imide dyes with a yellow green fluorescent dyes. However, the resulting durability of such articles is not discussed.
The third possible approach is for the polymer matrix to contain a blend of a non-fluorescent dye with a fluorescent dye. The issues noted above for multiple fluorescent dyes in the same polymer matrix are raised for this option as well. The issues could be even more difficult due to the typical greater chemical difference between a fluorescent dye and a non-fluorescent dye. Additionally, there is a chance that the non-fluorescent dye may interfere with the fluorescent properties of the fluorescent dye, which may dramatically reduce brightness of the sheeting. A non-fluorescent dye can quench the overall fluorescing of the fluorescent dye.
Accordingly, the current state of the art also is in need of a solution to this coloration problem. Typically, the provider of such articles does not have the ability to solve this coloration problem by dictating coloration standards to the end user of the fluorescent article. Instead, the end user typically dictates coloration to the manufacturer of such articles, and dye color availability is limited by dye suppliers. For example, governmental agencies, which would be the eventual end user of fluorescent highway road signs, will often define the color and/or durability standards for such signs.
It will be appreciated that attempting to address the two basic problems of light durability and coloration compliance within the same article increases the difficulties of these problems. Yet, a viable solution to these problems is all that more valuable when the same article successfully addresses both types of problems.